Abstract:

The self-organization of thin polymeric films triggered by an electric field is a new and intriguing phenomenon that offers a simple way to form periodic and hierarchical pillar-like structures without the help of a resist, exposure, and an expensively patterned mask. In this dissertation, we have developed theoretical models, simulated the process numerically, and performed extensive experiments to enhance our understandings to the fundamentals of this phenomenon, especially on its dynamic properties.

To answer the intriguing question of why hexagonal array was commonly observed in experiments, we developed a weakly nonlinear theory and found that the growth rate of the hexagonal pattern is faster than stripe or square arrays due to nonlinear interactions among Fourier modes of different orientations at the onset of symmetry-breaking. Two-dimensional simulations provided results that agree very well with experimental observations in terms of the periodic spacings and sizes of columns, and time for pattern formation. Moreover, it revealed several unique instability mechanisms and important process parameters under masks with different geometric patterns, which demonstrated rich phenomena in the nonlinear dynamics of this process and provided invaluable guidance for experimental designs.

With insights gained from the above studies, we integrated the conventional photolithography and the electro-induced self-organization to produce ordered patterns successfully over areas that are much larger than the natural domain sizes, a difficult task for most of the self-organization processes. A good variety of hierarchical structures have also been made, which reflects the versatility of this novel patterning method. We have also shown that the sizes, numbers, heights, spacings, and even packings of pillars in each unit cell can be easily tuned by a broad range of process parameters.

Interestingly, the periodic structures formed electrohydrodynamically are unstable and continued to evolve over a much longer time in our simulations. Neighboring pillars coalesce and the average size of the pillars increases until the thin residual layer on substrate renders further merging impractically slow. We found that the short time evolution of the interface is mainly governed by kinetics although the process is energetically driven overall. Thermodynamic preference takes over, however, at longer time and a coarsening phenomenon has been found. Both experiments and theory showed a logarithmic dependence of average pillar size on time. The scaling law is different from dewetting of thin films, primarily due to the significant effect of geometric confinement on the disjoining pressures.